1. Field of the Invention
The present invention relates to an image generating technology for aiding image diagnosis, and more particularly to a medical image generating device, method and program, that can generate an observation image suitable for diagnosis aid based on three-dimensional image data representing a spatial distribution of predetermined signal values (CT values and signal intensity) obtained with a diagnostic imaging apparatus such as a CT (computed tomography) apparatus, an MRI (magnetic resonance imaging) apparatus, or the like.
2. Description of the Prior Art
Medical image composition methods currently used in the medical field normally involve constructing on a computer a three-dimensional image model of a body to be observed on the basis of tomographic image groups obtained with a diagnostic imaging apparatus, and constructing then an image projected on a two-dimensional plane without losing the shape information of the three-dimensional image model. Known such methods include, depending on the type of image to be constructed, maximum intensity projection (MIP), minimum intensity projection (MinIP), surface rendering (SR) and volume rendering (VR).
In VR, a three-dimensional image model of a body to be observed is constructed by using an aggregation of image-constituting elements, called voxels, with which there are associated respective signal values such as CT values obtained with a diagnostic imaging apparatus. When obtaining a VR image, predetermined attributes (referred to hereinafter as “display properties”) required for imaging, such as color and degree of opaqueness, are given to each image-constituting element corresponding to each signal value (for example, see “MDCT and MRI of Cardiovascular Diseases” (Igaku Shoin)).
In VR display, appropriately setting the display properties allows grasping the three-dimensional structure of various sites, which is useful for roughly grasping the positional relationship between an affected site and bone or the surroundings of the affected site. In VR display, however, contrast information is likely to be missing in three-dimensional image data, which hampers visualization of low-contrast sites such as tapering blood vessels. Thus, it may become impossible to grasp the relationship between, for instance, an affected site and blood vessels linked therewith (blood vessels that supply nourishment to a tumor).
On the other hand, MIP is a method in which there are projected only those image-constituting elements that have the greatest signal values on the respective lines of sight set for the three-dimensional image model, and hence MIP is a method in which contrast information in the three-dimensional image data is reflected on a projection image, which allows visualizing even low-contrast blood vessels and the like, being thus advantageous for grasping a complete picture of the blood vessels.
In MIP display using contrast-enhanced CT images, in particular, blood vessels can be visualized down to the low-contrast portions where the vessels taper off, and thus the overall structure of the blood vessels can be grasped readily. Such MIP display is widely used, therefore, in wide areas such as the abdomen, thorax, lower limbs or the like.
However, when tissue such as bone, having higher signal values than blood vessels, is present in the area to be observed together with blood vessels, on the set line of sight, it is bone that becomes visualized, while the blood vessels fail to do so. For this reason there is carried out image processing in which portions corresponding to tissues such as bone or the like, having higher signal values than blood vessels, are extracted on the three-dimensional image model, and then the extracted portions are effaced through masking or the like (see, for instance, “Trade Secrets of Three-Dimensional Imaging and Processing Method for Medicine (Shujunsha)”).
However, the spine and the ribs, among others, offer important clues for determining the position of tissue to be observed, such as blood vessels, tumors or the like, in the abdomen and/or the thorax of the body. Expunging thus the spine, ribs or the like from images is hence problematic as this makes it more difficult to grasp the positional relationship of the tissue to be observed.